1. Field of the Invention
The present invention relates to a demodulator for a communication system employing an orthogonal frequency division multiplexing (OFDM) scheme, and more particularly, to a demodulator and a frame synchronization method for a communication system in which a communication is performed by an OFDM scheme using a time division multiple access (TDMA).
2. Description of the Related Art
In recent years, an orthogonal frequency division multiplexing (OFDM) communication system has been a focus of constant attention in a wireless communication system for its capability in a multipath environment and high frequency-use efficiency. Meanwhile, a time division multiple access (TDMA) communication system has better communication efficiency than a carrier sense multiple access (CSMA) that has been used in the conventional wireless local area network (LAN) because it accommodates user data in a frame without collisions of data. For this reason, an OFDM communication system using the TDMA has recently been commonly used instead of an OFDM communication system using the CSMA.
In the OFDM communication system using the TDMA, it is necessary to generate a frame pulse that indicates a head position of a reception frame at a receiver side. The frame pulse is a synchronization signal required for correctly demodulating an OFDM signal, which becomes a timing signal (FFT timing signal) for executing a fast Fourier transform (FFT). Therefore, in order to perform a demodulation in a correct manner, it is required to generate the frame pulse at just the right timing (right FFT timing). There are several methods to detect the right FFT timing; for example, a method of detecting the right FFT timing with an improved detection accuracy is disclosed in Japanese Patent Application Laid-open No. 2004-222207.
Furthermore, because there is a difference in the oscillation frequency between a clock oscillator of a transmitter and a clock oscillator of a receiver, the receiver in the OFDM communication system using the TDMA is required to have a clock-frequency synchronization function (a function of compensating a clock-frequency deviation). In other words, the receiver is required to have two synchronization functions including a function of generating the frame pulse (an FFT-timing synchronization function) and the clock-frequency synchronization function.
If the clock-frequency deviation is large, the orthogonality cannot be maintained between subcarriers of the OFDM, and as a result, the quality of demodulated data is degraded. Therefore, a standard has set a usage of a high-accuracy clock oscillator, without a necessity of considering an influence of the clock-frequency deviation because is ignorable. In short, the conventional communication system only requires the FFT-timing synchronization function as the synchronization function without putting a particular consideration about the clock-frequency deviation (see, for example, Japanese Patent Application Laid-open No. 2004-222207).
The accuracy level required in a clock oscillator increases as the FFT data size increases. With a background of a demand in high speed against the recent wireless communication, the FFT data size tends to increase compared to the conventional wireless LAN, which requires a clock oscillator with a higher accuracy. However, the high-accuracy clock oscillator is expensive so that it increases the cost of a whole apparatus. In order to avoid a cost rise, it is necessary to employ a cheap and low-accuracy clock oscillator, although in this case, a clock-frequency deviation is possibly generated which is not ignorable (sufficiently large).
However, the conventional receiver has no synchronization function considering a large clock-frequency deviation. As a result, if there is a large clock-frequency deviation, the frame synchronization function cannot establish a correct synchronization or a following performance after a synchronization is degraded, which is problematic.